Inertial navigation of a platform is based upon the integration of specific forces and angular rates as measured by inertial sensors (e.g. accelerometer, gyroscopes) of a device containing the sensors and positioned within a motion-capable platform. In traditional systems, the device is tethered to the platform. Measurements from the device may be used to determine the position, velocity and attitude of the device and/or the platform.
Alignment of the inertial sensors within the platform (i.e. alignment of the device containing the sensors with the platform's forward, transversal and vertical axis) is typically required for traditional inertial navigation systems. Where the inertial sensors are not properly aligned, the positions and attitude calculated using measurements from the inertial sensors will not be representative of the state of the platform. As such, in order to achieve high accuracy navigation solutions, inertial sensors must be tethered within the platform and careful manual mounting of the device within the platform is needed.
Portable navigation devices (or navigation-capable devices), however, are able to move, whether constrained or unconstrained within the platform (such as for example a person, vehicle, or vessel of any type), and careful mounting or tethering of the device to the platform is not an option.
As navigation-capable devices (e.g. mobile/smart phones) become increasingly popular, they can come equipped with Assisted Global Positioning System (AGPS) chipsets having high sensitivity capabilities capable of providing absolute positioning of the platform (e.g. user) even in environments without a clear line of sight to satellite signals. In environments where AGPS information alone is not enough, such as deep indoors or in challenging downtown navigation or localization, one possible solution is to incorporate cell tower identification or, if possible, trilateration of cell towers for a position fix (where AGPS solution is unavailable). Despite these two known positioning methods available in many mobile devices, accurate indoor localization still presents a challenge and fails to satisfy the accuracy demands of current location based services (LBS). Additionally, these methods may only provide the absolute heading of the platform, without any information on the device's heading.
Mobile navigation-capable devices (e.g. mobile/smart phones) can come equipped with Micro Electro Mechanical System (MEMS) sensors that are used predominantly for screen control and entertainment applications. These sensors have not been broadly used to date for navigation purposes due to very high noise, large random drift rates, and frequently changing orientations of the device with respect to the platform.
Mobile devices can also come equipped with magnetometers, and in some cases, it has been shown that a navigation solution using accelerometers and magnetometers may be possible if the user is careful enough to keep the device in a specific orientation with respect to their body, such as when held carefully in front of the user after calibrating the magnetometer.
There is a need, however, for a method of providing a navigation solution that is capable of accurately utilizing measurements from a navigation-capable device within a platform, and thereby determining the navigation state of the device/platform without any constraints on the platform (i.e. in indoor or outdoor environments) or the mobility of the device within the platform. The estimation of the position and attitude of the platform should be independent of the usage of the device (e.g. the way the user is holding or moving the device during navigation).
There is also a need for such a method to allow for the device to be tilted in any orientation, including vertical or near vertical orientations, while still providing seamless navigation information without degradation in performance. Difficulties arise when a mobile navigation-capable device becomes vertical or near vertical because azimuth (also called heading) and roll of the device can have large variations, which can also cause a large variation in the azimuth estimation of the platform. It is understood that the large variations in the azimuth and roll of the device result because of the definition of pitch, roll, and azimuth. Pitch angle is from −90 degrees to 90 degrees, while azimuth and roll are each from −180 degrees to 180 degrees.
The azimuth angle is the heading clockwise from North direction in the East-North plane, the pitch angle of the device is the angle between the forward axis of the sensor frame of the device and the East-North plane, the roll angle is the angle around the forward axis of the device. When the pitch angle goes towards 90 degrees or −90 degrees and because of the definition especially the discontinuity in the pitch angle as defined, both the roll and azimuth tend to represent angles in the East-North plane as the forward axis of the device becomes vertical. Furthermore, if the pitch angle changes from 80 degrees towards 90 degrees, the angle will go back towards 80 degrees at the point where it wanted to cross the 90 degrees. The same happens if the pitch angle changes from −80 degrees towards −90 degrees, that is—the angle will go back towards −80 degrees at the point where it wanted to cross the −90 degrees. As a result, the azimuth and roll tend to flip by 180 degrees, which causes problems to navigation solutions using the sensors in the device.
Thus, a method for mitigating the above described problem when the pitch angle of the device approaches near 90 or −90 degrees is required.